Bearings used for rotating devices have a long history of development. However, special applications for bearings present problems not satisfactorily solved by previous devices. Such special applications include applications which require high rotational speed, high pressure operation, high load capacity, close tolerance and/or operation on a low viscosity fluid. Examples of such special applications include high-speed turbo pumps such as rocket propellent pumps or other fluid pumps which may be used, for example, in the National Aerospace Plane (NASP), the Orbital Transfer Vehicle (OTV), and the Space Shuttle main engine (SSME).
Bearings which have been used in high-speed applications have included ball bearings, plane hydrodynamic bearings, hydrodynamic foil bearings and separate thrust and journal hydrostatic bearings. As an example, in one design for a high pressure fuel pump of the Space Shuttle main engine, ball bearings were placed at the outboard ends of the shaft to accommodate bearing speed limitation (DN) limits. This design led to a supercritical rotor design which, in turn, resulted in an unstable rotor which was subject to destructive and unpredictable subsynchronous whirl.
The foil bearing has inherently low stiffness (e.g., 1.times.10.sup.5 lb/in, about 1.8.times.10.sup.5 N/cm, for applications such as the special applications described above) and large deflection (0.010 in, about 0.25 mm) due to the compliant foils. The foil bearing also has low load capacity (i.e., unit loading less than 200 psi, about 1350 kPa) due to hydrodynamic action in low viscosity fluid. These characteristics are similar to those which led to subsynchronous whirl in the Space Shuttle main engine (SSME) fuel pump, described above.
Separate thrust and journal hydrostatic bearings have improved load capacity and stiffness and lower speed limitations compared to ball bearings and hydrodynamic foil bearings. However, for high-speeds, providing high stiffness also requires a very close radial clearance. This close radial clearance cannot practically and economically be obtained by contemporary machining. Furthermore, separate hydrostatic thrust and journal bearings in a high-speed application provide a device which is prone to variation of clearance with speed. This variation has been found to lead to unacceptable performance particularly in the special applications described above.
Certain previous devices have provided bearings which have a surface defining a portion of a sphere. For example, U.S. Pat. No. 2,998,991, issued Sep. 5, 1961 to Morser, et al., provides bearings which have clearances that range between 0.001 inches and 0.005 inches (about 0.0025 mm to about 0.127 mm). Typically, such previous devices have not been designed to provide the performance necessary for the special applications described above. Such previous devices are not designed to provide the extremely high rotation rates typically used in connection with low viscosity fluids. For example, at rotation rates greater than about 100,000 rpm and shaft surface radii of about 1.3 inches (about 33 mm) and clearances of about 0.001 in (about 0.025 mm), centrifugal forces tend to significantly distort the shape of portions of the shaft. The problems presented by such distortion are particularly acute in devices with extremely small clearances between the shaft and its bearing (e.g., about 0.0003 inches, about 0.0075 mm). Previous spherical bearings were not configured to accommodate such centrifugal growth.